The present invention relates to a circuit for heating a component.
German Patent No. 195 31 786 describes a circuit arrangement for activating a heating resistor that is provided, in particular, for heating an air quality sensor. The temperature of the heating resistor is regulated to a predefined value. The heating resistor is activated with a two-position temperature controller. If the reference temperature and actual temperature differ from one another, current flow through the heating resistor then occurs. The actual temperature of the heating resistor is determined indirectly by sensing its electrical resistance. The heating resistor is a constituent of a voltage divider through which, during the switched-off phase of the heating current, there flows a small current that allows a conclusion to be drawn, via the voltage drops in the voltage divider, as to the internal resistance of the heating element.
Another circuit arrangement for temperature regulation of a resistance heating system has been described in German Patent No. 43 12 289. With this conventional circuit arrangement, continuous closed-loop control of the heating output is provided instead of a two-position control system. Here again, the electrical resistance of the resistance heating element, which is arranged in a voltage divider, is sensed. The difference between the voltage drops occurring, on the one hand, at a known resistance and, on the other hand, at the resistance heating element is evaluated. The magnitude of the difference defines the operating voltage of the voltage divider containing the resistance heating element.
The known resistance heating elements are provided for heating of sensors which must be at an operating temperature higher than ambient temperature in order to work correctly. In the case of the conventional circuit arrangements, it is assumed that the operating temperature of the sensor always corresponds at least approximately to that of the resistance heating element.
It is an object of the present invention to describe a circuit for heating a component which indicates, with simple means, the fact that the operating temperature of the component to be heated has been reached.
A temperature of the component to be heated is not necessary. The fact that an operating temperature has been reached is deduced exclusively from the operating behavior of the resistance heating element. According to the present invention, at least one observation time interval is defined, within which a determination of the average energy delivered to the resistance heating element is provided. The fact that the average energy falls below a predefined threshold is interpreted as meaning that the predefined operating temperature of the component to be heated has at least approximately been reached. The present invention makes use of the fact that the energy to be delivered to the heating element during the operation of heating up the component to be heated is higher than in the steady state operating condition, in which the component to be heated has reached the operating temperature.
The average energy threshold to be ascertained is preferably determined experimentally. In the context of series production, an individual definition of the threshold for each component to be heated can be performed at the end of the line.
It is particularly advantageous to use a two-position controller for temperature regulation of the resistance heating element. The two-position temperature controller compares the actual temperature occurring at the resistance heating element to the predefined reference temperature, and, as a function of the result, either connects the resistance heating element to an energy source or interrupts the connection.
An advantageous development of this embodiment according to the present invention provides for the switched-on time of the two-position temperature controller within the observation time interval to be determined. The switched-on time is a direct indication of the average energy delivered to the resistance heating element. Instead of an integration of the energy delivered to the resistance heating element, all that is to be provided is thus a determination of a duration. What is to be defined in this case as the threshold of the average energy is a maximum duration within the observation time interval.
Another advantageous development of this embodiment according to the present invention provides for the number of switch-on events of the two-position temperature controller within the observation time interval to be determined. Once the component to be heated has been heated up, the two-position temperature controller reduces the average electrical energy delivered to the resistance heating element by switching on and off more frequently. In this case, the minimum threshold for the average energy corresponds to the definition of a number of switch-on events which must at least be attained within the observation time interval.
Another advantageous embodiment of the circuit according to the present invention provides for the internal resistance of the resistance heating element to be evaluated as an indication of its temperature. As a result, a separate temperature sensor for sensing the temperature of the resistance heating element is not necessary. The internal resistance of the resistance heating element can be determined using one of the actions explained in the documents cited in the xe2x80x9cBackground Informationxe2x80x9d section.
A sensor is preferably provided as the component to be heated. The sensor can be, for example, a gas sensor which converts the constituents of a gas being investigated into a corresponding output signal. Sensors of this kind are used, for example, in determining the gas constituents of internal combustion engine exhaust gases, or ascertaining the quality of inhaled air by evaluating at least the CO concentration and NOx concentration in the inhaled air.